This invention relates to an automatic leaching system for the hydrometallurgical production of zinc, and more particularly to a neutral leaching section in such a system.
In a leaching system for the hydrometallurgical production of zinc, it is necessary to control the pH value of the slurry so that it can be maintained at a predetermined setting. It is very important to improve the reliability of this pH control since the effect of leaching as well as the effect of later purification is affected greatly by the effect of control of the pH value. A double continuous leaching system is commonly employed in the art. This system consists of a neutral leaching section in which calcine is leached with a solution overflowing from an acid thickener to obtain a slurry, and an acid leaching section in which a spent electrolyte is added to the slurry for further promoting the leaching reaction. However, reliable pH control is difficult to attain in the neutral leaching section due to the fact that various variable factors which should be taken into account are involved in the operation of this neutral leaching section. Especially when this pH control is carried out manually, further difficulty is encountered in attaining the desired reliable pH due to the fact that another variable factor owing to the difference in the skill of the operators is added to the variable factors above described. An example of this manual pH control is shown in FIG. 7. It will be seen from the graph of FIG. 7 that the pH value of the slurry varies greatly within the range of about 3.0 to 5.0, and thus, the reliability of the pH control is quite low. An automatic pH control has been proposed to an effort to obviate the difficulty encountered with such manual control. In this automatic control, there is such an essential requirement that the pH value must be continuously and automatically measured. However, the proposed automatic control has been defective among others in that continuous automatic measurement of the pH value cannot be attained due to a large amount of scale tending to adhere to the surface of the electrodes of the pH meter. Various devices have been proposed hitherto in an effort to remove the scale adhered to the surface of the electrodes of the pH meter. One of the prior art devices utilizes ultrasonic waves for the automatic washing of the electrodes, and another utilizes a washing brush adapted for periodic splash washing of the electrodes by water. However, these prior art devices have not been so effective in slipping down the scale resulting in incapability of reliable automatic measurement of the pH value. Thus, the prior art efforts have failed in automating the leaching process for the hydrometallurgical production of zinc.